8 - PHOTORECEPTORS Flashcards
Where does light enter through?
The pupil
What controls the amount of light that enters the eye?
The muscles of the iris
What are light rays focused by?
The lens of the retina
What detects light?
The photoreceptor cells
Where are a lot of photoreceptors located?
The fovea
What carries the nerve impulses the photoreceptor cells from the retina to the brain?
The optical nerve which is a bundle of neurones
What is the blind spot and what doesn’t it contain?
Where the optic nerve leaves the eye.
It doesn’t have photoreceptor cells so its not sensitive to light.
Check image on page 176
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How do photoreceptors convert light into an electrical impulse?
1) Light enters the eye, hits the photoreceptors and is absorbed by light-sensitive pigments
2) Light bleaches the pigments causing a chemical change
3) Bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain
What are the two types of photoreceptor and what are the differences?
- Rods are mainly found in the peripheral parts of the retina and give information in black and white, and cones are only packed together in the fovea and give colour.
How many different types of cones are there?
there are three:
1) Red sensitive
2) Green sensitive
3) Blue sensitive
What are rods made of?
Two chemicals joined together - retinal and opsin
What happens when it’s dark and the rods aren’t stimulated?
1) Sodium ions (Na+) are pumped out the cell using active transport
2) But sodium ions diffuse back in to the cell through open sodium channels
3) This makes the inside of the cell only slightly negative compared to the outside (cell membrane is depolarised)
4) This triggers the release of neurotransmitters
5) But the neurotransmitters inhibit the bipolar neurone- so the bipolar neurone can’t fire an action potential so no information goes to the brain
What happens when its light and the rods are stimulated?
1) Light energy causes rhodopsin to break apart into retinal and opsin - this process is called bleaching
2) The bleaching of rhodopsin causes the sodium ion channels to close
3) So sodium ions are actively transported out of the cell, but they can’t diffuse back in
4) This means sodium ions build up on the outside of the cell, making the inside of the membrane much more negative than the outside - the cell membrane is hyperpolarised
5) When the rod cell is hyperpolarised it stops releasing neurotransmitters. Meaning that there is no inhibition of the bipolar neurone.
6) Because the bipolar neurone is no longer inhibited, it depolarises. If the change in potential difference reaches the threshold, an action potential is transmitted to the brain via the optic nerve
